I am also now learning on a 1989 myself. It is just now recovering from a near death experience from heavy root pruning. You are right about the vigorous growth - once it decided to live it went gangbusters.
There is a strong tendency in this cultivar to multiple shoots from one spot. The inverse taper is unfortunate, but fortunately you've got fantastic taper drown low. The problem up high will never resolve in zone 5 (?). So if this were mine, I would cut back by at least a third, to a new leader, and wire that up to grow a new top.
These seem to die back badly from chops, so I suggest a flat cut at first, waiting to cut diagonally until that dries up. As for the chopped-off piece, I would cut it again, higher, to eliminate the reverse taper then plant that. It could turn out as a really nice shohin.
It will root for sure, but it will need warmth and VERY high humidity. Trim branches back to a few leaves each, but don't leave it leafless. Wait a few weeks and look for buds opening.
Oh, and yeah, all of this will go a lot better if you do it in the summer. It's great you are seeing all that growth under lights. You will need that to grow a healthy root mass. By the time it goes outside next summer, you should be well set up for the next steps above.
Good luck!